As is well known, information about the level of coins in coin tubes may be used, among other things, for the purpose of controlling the delivery of tested and accepted coins to the tubes, and the dispensing of coins from the tubes, so as to avoid the problems of attempting to over-fill a tube, which would cause jamming, and attempting to dispense from an empty tube.
In the applicants' EP-B-0017428 there was disclosed an optical sensor which has proved successful and been widely used, in which a light beam from a light source crosses the tube, is internally reflected twice at the wedge-shaped end portions of a trapezoidal prism, so as to turn the beam through 180.degree., and returns across the tube to a light detector
For the purpose of the present specification the term "light" is not of course confined to the optical part of the spectrum.
The above-mentioned arrangement has certain advantages, such as the folded light beam covering a larger area than a straight beam so as to more reliably sense coins which occasionally are at an angle within the tube, and the fact that the source and detector can be at the same side of the tube so that electrical connections can be made from one side only. The prism can be fitted to, or built into, the tube itself.
It has been found, however, that such detectors have limitations which become more severe as the total length of the path of the light beam from the source to the detector increases. In particular, the power available from the beam for activating the detector falls, and this is aggravated by the fact that small relative misalignments of the source, prism and detector further reduce the power that the detector actually receives.
The first of these problems can be reduced by increasing the power input to the source, but this reduces the useful lifetime of the source itself. The second problem can be reduced by increasing the size of the internally reflecting end faces of the prism, so as to increase the area of the light beam that can traverse the system, but this involves making the prism not-only wider, but also deeper, so that it starts to take up an unacceptable volume within the coin mechanism, where compactness is desired. Further disadvantages of such detectors are that light is lost from the beam where it is transmitted through two surfaces of the prism, where it is reflected at two other surfaces of the prism, and also during its transmission through the material of the prism, which further reduces the power available to activate the detector; and, for a given prism size, the area of the beam that can be reflected through 180.degree. is less than half the area of the entry and exit face of the prism because of the need for two independent reflection steps.